A pressure vessel is a container to withstand a pressure differential between inside and outside. A pressure vessel may comprise of multiple layers of materials to provide various required properties. The inside and outside of the vessel may have special properties to meet the environment while the inner layers may be designed to provide strength, fatigue and fracture toughness to the vessel. The design is governed by standard pressure vessel equations published in books such as Harvey (see reference), and standards such as ASME Section VIII, Div. I, DOT FMVSS 304, ANSI-NGV-2, ISO11439 and several others. As an example, Dallum in U.S. Pat. No. 5,024,342 has shown a multilayer container to improve corrosion resistance.
In a thin wall cylindrical vessel the transverse stress is exactly double to that of the longitudinal stress. High strength wires therefore provide advantage in reinforcing a pressure vessel where the stresses are not uniform in each direction. In 1918, Goodall, U.S. Pat. No. 1,281,557 used steel wire of round or rectangular shapes to reinforce a rubber matrix hose. A textile fabric was used under and over the wires. Composite Overwrapped Pressure Vessels (COPV) using glass or carbon fibers have been in use for decades for very light weight applications. However, there has been a challenge in transferring the tensile strength of the reinforcing fibers to the vessel structure. Several reports show that on average 80% and 60% of the carbon filament strength is transferred to the pressure vessel at 35 MPa and 70 MPa design pressure tanks respectively. One of these reports is entitled, “Low Cost, High Efficiency, High Pressure Hydrogen Storage, DoE Review, Feb. 8, 2005, by Mark J. Warner, Quantum. It is believed that this significant reduction is related to the pre-tension, lay up arrangement and the sharp drop in off-axial properties of glass and carbon fibers which have zero plastic ductility.
Cord wrapped vessels have several advantages which include reduction of weight and cost. An individual filament has anisotropic properties. The strength, modulus, hardness etc. along longitudinal axis of the filament far exceeds that in the transverse direction. This makes the filament very strong along the longitudinal axis. Glass, carbon or steel filaments along the longitudinal axis typically have strength in the 3000 MPa to 6000 MPa range.
Composites made with longitudinal filaments, however, have lower mechanical properties along the transverse axis. As an example CYPLY® 1002 is a cured epoxy composite material based on a unique non-woven parallel filament construction in a polymer matrix. Tensile strengths at 0°, along 45° and 90° of fiber axis are 965, 24 and 20 MPa respectively. Even composites made of cross woven non ductile fibers such as fiberglass, e.g. FR-4 G-10 show significant anisotropy in mechanical properties.
When the cylinder winding filaments are steel wire or cords, the cut ends of wires have significant stiffness and do not conform to the curvature of the liner. This causes a problem which Steiner in U.S. Pat. No. 4,113,132 proposed welding the wire ends to the adjoining wires. This works in heavy gauge wires, however, is impractical in thin high carbon wires. Spun end pressure vessels are superior in pressure holding capacity precisely due to no use of welds in the cylinder body.
Once wrapped, the cylinder is still exposed to external damage and degradation. Most composite pressure vessels have a top layer of protective material to absorb damage from outside. Foam, other composites or similar materials have frequently been used. DeLay in U.S. Pat. No. 6,953,129 has discussed methods to apply a damage and/or fire tolerant outer layer. The layers comprise of jute and other strong fibers and microspheres containing a temperature responsive phase change material. DeLay in U.S. Pat. No. 7,641,949 suggests blending a high toughness fiber with the high strength fiber to gain surface toughness. Long in U.S. Pat. No. 4,844,287 has proposed using a liquid transmissive textile, and an outer polymeric containment layer. The liquid transmissive geotextiles may be woven, knit or non-woven fabrics or a needle punctured fabric. U.S. Pat. No. 5,476,189 titled Pressure vessel with damage mitigating system by Duvall, et al cites the use of a foam or crushable material near the outside of the vessel. Bastone in U.S. Pat. No. 3,412,891 suggests the use of a woven stabilizing scrim material to stabilize resin rich material on a vessel.
Light weight vessels are used for Self Contained (Underwater or regular) Breathing Apparatus, storage of gases on board an aircraft or other vehicles, or storage of gaseous fuels such as hydrogen or Compressed Natural Gas (CNG) on board a vehicle. Light weight vessels are made of very high strength metals such as aluminum, titanium or steel and may be circumferentially or fully wrapped with a reinforcing high strength cord of Aramid, fiberglass, carbon, steel etc. One version of the light weight vessels is called a composite over wrapped pressure vessels (COPV) which is usually wrapped with high strength glass or carbon fibers which are completely embedded in a polymer resin matrix.
The present invention teaches a novel solution to the problem of starting and finishing wire winding and maximizing the efficiency of the wire winding.
The present invention additionally cites a novel solution to the problem of adhering the cut wire ends without adversely affecting the structural integrity of the cylinder or materially increasing the weight of the finished cylinder.